if any page on your site falls outside of these categories, the banners aren't targeted anymore. And what that means is that you're potentially showing ads to people who don't have any interest in them and that can lead to inefficiencies.
With Google AdSense that just about never happens: the ads present on your site will always be in sync with whatever your visitors are interested in and that adds to your site's value and to the revenue it produces.
What's more, the look of these banners and their size is customizable, which means they will feel more integrated with the rest of your site's content, which brings an increase in your site's overall visual quality as opposed to a traditional approach.
For many people, there's also a great problem with finding people to advertise on their site as well. AdSense is clearly the easiest solution available to this problem today.
It's free to join the AdSense program and it almost takes no time at all. The potential database of websites from AdWords is larger then anything you might encounter from any competitor, numbering over 150.000 users. This means as more people compete the CPC or cost per click for search terms will inevitably rise.
Also, setting up AdSense on your site is a breeze, and you can complete the whole process in less then an hour. It takes a lot less to do then any form of affiliate advertising which is yet another reason to choose the easy AdSense approach.
All you need to do is add an AdSense banner on your website (other then registering with the program of course) is to copy & paste a few lines and you're all done, you get to choose where the banners go, how large they are and how they integrate with the rest of your site.